The present disclosure relates to the field of magnetic bearings. In particular an apparatus and method for reducing stray transverse magnetic fields caused by permanent magnetic bearing arrangements in high speed rotary machines such as turbomolecular pumps.
Turbomolecular pumps are employed as part of the vacuum system used to create the high vacuum environment required for devices such as scanning electron microscopes (SEMS) and lithography devices.
It is common for turbomolecular pumps to comprise an oil free passive permanent magnetic bearing arrangement in the high vacuum end of the pump.
A cross section of a passive permanent magnetic bearing arrangement 10 for a turbomolecular pump (not shown) is illustrated in FIG. 1. In this example the bearing arrangement 10 comprises an array 12 of four outer rotating permanent magnet rings 12a, 12b, 12c and 12d and an array 14 of four inner non-rotating permanent magnetic rings 14a, 14b, 14c and 14d arranged such that the outer, rotating, array 12 surrounds the inner, static, array 14 in a concentric manner. The magnets are all formed of rare earth magnetic material. The outer array 12 is attached to the rotor of a turbomolecular pump (not shown) with the static array 14 attached to the stator of said pump. For reasons of mechanical strength and practical construction, it is normal for the outer array of rings to form the rotating part of the bearing arrangement and the inner rings to form the stationary part.
In this example the magnetisation of the magnetic rings 12a to 12d and 14a to 14d in each array 12, 14 respectively is substantially aligned with the axis of rotation 4 of the pump rotor (not shown). The direction of magnetisation has been indicated by the arrows, with the head of each arrow indicating the north pole.
The magnets are arranged within each array such that they are in mutual repulsion with each other; that is proximate magnets in an array meet their nearest neighbouring magnet in the same array with the same pole (e.g. magnets 12a and 12b meet each other with their south poles). The outer magnetic rings 12a, 12d, 14a, 14d in each array have their north poles facing outermost.
The magnets 12a to 12d and 14a to 14d in each array 12, 14 of the arrangement 10 are orientated to provide a mutual repulsion between the arrays 12, 14 and therefore create an almost frictionless bearing.
A great many other configurations are possible, using different numbers of rings, with axial or radial magnetisation, and arranged for either repulsive or attractive forces between rotor and stator. Although a variety of configurations are possible, they all perform optimally when the direction of magnetisation in the rings is perfectly symmetrical with respect to their geometric axis.
The magnetisation in the rings 12a to 12d of the rotating array 12 is shown in FIG. 1 as perfectly symmetrical with respect to their geometric axis 4. However, in reality, the magnetisation of each magnetic ring 12a to 12d (and, similarly, for magnets 14a to 14d) is imperfect due to the practical limitations of their manufacturing process. This is illustrated in FIGS. 2a and 2b. The largest magnetic asymmetry observed in axially magnetised permanent magnetic rings is usually a small angular error such that the magnet's axis is displaced from the geometric axis 4 by an angle of a few degrees as indicated in FIG. 2a. Depending on the quality, or grade, of the magnet the angular error, θ, can be as much as 3°. This error may be regarded as a small perturbation from the ideal axial magnetisation; in effect a transverse magnetic dipole moment 8 superimposed on the intended axial dipole moment 6 as illustrated in FIG. 2b. 
In addition to the transverse dipole (first order) asymmetry, higher order asymmetries exist, for example quadrupole and hexapole asymmetries. The magnitude, or magnetic field strength, of the asymmetry usually decreases as the number of poles increases.
Where these small asymmetries occur in any of the rings 12a to 12d of the rotating magnet array 12, a time varying magnetic field is generated (the magnetic field is constant for the static magnets 14a to 14d). These 2, 4 and 6 pole asymmetries generate time varying magnetic fields at frequencies of 1, 2, 3 times the rotational speed of the pump rotor respectively.
The performance of scanning electron microscopes is highly susceptible to mechanical vibrations or stray magnetic fields emitted from turbomolecular pumps. The stray fields are known to directly interfere with the electron beam or with the instruments' electrical circuits.
Although it common to use ferromagnetic shielding to reduce such magnetic field emissions, such shielding is costly and is only of limited effectiveness.
Therefore it is desirable to reduce the effect of these time-varying stray magnetic fields by alternative means.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.